The present invention relates to implantable medical devices such as cardiac monitoring devices and rate-responsive pacemakers. More particularly, this invention is directed toward a pyroelectric suppressor circuit that prevents undesirable thermally induced signals generated by piezoelectric activity sensors from reaching processing circuitry within such devices.
A pacemaker is an implantable medical device which delivers electrical stimulation pulses to cardiac tissue to relieve symptoms associated with bradycardia--a condition in which a patient cannot normally maintain a physiologically acceptable heart rate. Early pacemakers delivered stimulation pulses at regular intervals in order to maintain a predetermined heart rate--typically a rate deemed to be appropriate for the patient at rest. The predetermined rate was usually set at the time the pacemaker was implanted, although in more advanced pacemakers, the rate could be set remotely after implantation.
Early advances in pacemaker technology included the ability to sense the patient's natural cardiac rhythm (i.e., the patient's intracardiac electrogram, or "IEGM"). This led to the development of "demand pacemakers"--so named because they deliver stimulation pulses only as needed by the heart. Demand pacemakers are capable of detecting a spontaneous, hemodynamically effective cardiac contraction which occurs within a predetermined time period (commonly referred to as the "escape interval") following a preceding contraction. When a naturally occurring contraction is detected within the escape interval, the demand pacemaker does not deliver a pacing pulse. The ability of demand pacemakers to avoid delivery of unnecessary stimulation pulses is desirable because by doing so, battery life is extended.
Demand pacemakers allow physicians to telemetrically adjust the length of the escape interval, which has the effect of altering the heart rate maintained by the device. However, in early devices, this flexibility only allowed for adjustments to a fixed programmed rate, and did not accommodate patients who required increased or decreased heart rates to meet changing physiological requirements during periods of elevated or reduced physical activity. Therefore, unlike a person with a properly functioning heart, a patient receiving therapy from an early demand pacemaker was paced at a constant heart rate--regardless of the level to which the patient was engaged in physical activity. Thus, during periods of elevated physical activity, the patient was subject to adverse physiological consequences, including lightheadedness and episodes of fainting, because the heart rate was forced by the pacemaker to remain constant.
The adverse effects of constant rate pacing lead to the development of "rate-responsive pacemakers" which can automatically adjust the patient's heart rate in accordance with metabolic demands. An implanted rate-responsive pacemaker typically operates to maintain a predetermined minimum heart rate when the patient is engaged in physical activity at or below a threshold level, and gradually increases the maintained heart rate in accordance with increases in physical activity until a maximum rate is reached. Rate-responsive pacemakers typically include processing and control circuitry that correlates measured physical activity to a desirable heart rate. In many rate-responsive pacemakers, the minimum heart rate, maximum heart rate, and the transition rates between the minimum heart rate and the maximum heart rate are parameters that may be adjusted to meet the needs of a particular patient.
One approach that has been considered for enabling rate-responsive pacemakers to determine an appropriate heart rate involves measuring a physiological parameter that reflects the level to which the patient is engaged in physical activity. Physiological parameters that have been considered include central venous blood temperature, blood pH level, QT time interval and respiration rate. However, certain drawbacks (such as slow response time, unpredictable emotionally-induced variations, and wide variability across individuals) render the use of these physiological parameters difficult, and accordingly, they have not been widely used in practice.
Rather, most rate-responsive pacemakers employ sensors that transduce mechanical forces associated with physical activity. These activity sensors generally contain a piezoelectric transducing element which generates a measurable electrical potential when a mechanical stress resulting from physical activity is experienced by the sensor. By analyzing the signal from a piezoelectric activity sensor, a rate-responsive pacemaker can determine how frequently pacing pulses should be applied to the patient's heart. Piezoelectric activity sensors can also be used in implantable medical devices that serve solely as cardiac monitoring devices.
Piezoelectric elements for activity sensors are commonly formed from piezoelectric ceramics, such as barium titanate. Recently, however, activity sensors have been designed which use thin films of a piezoelectric polymer, such as polyvinylidene fluoride (commonly known by the trademark KYNAR, owned by ATOCHEM North America) as the transducing element, rather than the more commonly used piezoelectric ceramics. Activity sensors so designed are described in commonly-assigned U.S. Pat. Nos. 5,383,473, entitled "Rate-Response Implantable Stimulation Device Having a Miniature Hybrid-Mountable Accelerometer Based Sensor and Method of Fabrication," and 5,425,750, entitled "Accelerometer-Based Multi-Axis Physical Activity Sensor for a Rate-Responsive Pacemaker and Method of Fabrication," now U.S. Pat. No. 5,425,750, which are hereby incorporated by reference in their entireties.
The activity sensors described in the above-incorporated patent applications, which use a resilient piezoelectric polymer as the transducing element, offer significant advantages over sensors which use piezoelectric ceramics. These advantages are largely attributable to the resiliency of the thin polymer films. For example, the piezoelectric polymer films are better able to withstand stresses that may occur during sensor fabrication, thereby reducing the cost and complexity of the fabrication process. In addition, activity sensors which use the polymer films may be designed to respond more aggressively to mechanical stresses resulting from physical activity, so that they provide stronger output signals. Indeed, the output potentials provided by activity sensors that use polyvinylidene fluoride transducing elements typically have magnitudes of about 200 mV (RMS), whereas piezoelectric crystal sensors provide output potentials which typically have magnitudes of just a few mV (RMS).
Despite the advantages associated with the use of piezoelectric polymer films in activity sensors, an unexpected difficulty has been encountered. It is known that piezoelectric materials exhibit a pyroelectric effect. More precisely, temperature fluctuations can induce mechanical stresses in piezoelectric materials, thereby causing the material to generate output potentials in response to the temperature changes. Because they are typically implanted superficially beneath the skin, implantable medical devices may often experience temperature fluctuations on the order of about 0.1.degree. C./minute, and an overall temperature range of about 4.degree. C. is not unusual. However, piezoelectric ceramics tend not to respond too significantly to temperature fluctuations commonly experienced by implantable medical devices. Therefore, the performance of an implantable medical device that uses a piezoelectric ceramic activity sensor would not ordinarily be adversely affected by thermally induced stresses.
However, it has been found that activity sensors which use piezoelectric polymer films exhibit a more pronounced pyroelectric effect--to an extent that may have an impact on the performance of the implantable medical devices which use such sensors. Thermally induced stresses in piezoelectric polymer sensors may cause such sensors to generate signals as large as 10 mV, which can represent a significant fraction of the total output. Thus, in the rate-responsive pacing context, a patient may experience a noticeable increase in heart rate during a bath or shower, for example, despite there being no change in activity level.
What is needed therefore is an implantable medical device which is capable of suppressing thermally induced signals generated by a piezoelectric activity sensor, so that such signals are not improperly interpreted as being indicative of physical activity. Preferably, the amount of additional circuitry required to suppress thermally induced signals should be kept to a minimum, so that the size and cost of the implantable medical device do not become prohibitive.